The aim of this proposal is to develop a non-myeloablative conditioning protocol with increased safety and efficacy for the induction of specific immune tolerance to genetically modified cells. Potential applications of gene therapy in the treatment of genetic and autoimmune disorders are widespread, but progress has suffered from a severe reduction in efficacy due to immune reactions raised against therapeutic transgene products. Induction of specific immune tolerance through mixed hematopoietic chimerism is an established methodology that can effectively re-educate the immune system to eliminate immune rejection. To achieve engraftment of allogeneic stem cells, conditioning requirements are associated with toxicities, and graft-versus-host disease is a common side effect. Studies have shown that molecular chimerism can be used to induce tolerance. Our preliminary data suggest that specific immune tolerance can be induced to a single immunogenic protein expressed on autologous cells, and that conditioning requirements for transplantation of molecularly modified autologous cells are less than those required for allogeneic transplants. We believe that the effectiveness of gene therapy can be greatly improved by inducing specific immune tolerance to foreign proteins to be transferred and that this can be accomplished using non-toxic protocols.